Tests that set `target.process.follow-fork-mode = child` crash when computing the execution context on the event thread

[JDevlieghere]

The tests that set target.process.follow-fork-mode = child crash when computing the execution context on the event thread. I'm observing this in the context of #148994 and #159887.

The tests fail consistently in CI and relatively reliably when running the test suite in a Linux VM. However, running the exact same lldb invocation outside the test never reproduces the issue (not within 200 runs) and neither does it trip up ASan, UBSan or TSan.

The issue is reproducible by calling Debugger::GetSelectedContext from the default event handler.

"Pretty" backtrace:

 #0 0x0000000000264268 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Support/Unix/Signals.inc:834:13
 #1 0x0000000000261c75 llvm::sys::RunSignalHandlers() /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Support/Signals.cpp:105:18
 #2 0x0000000000265021 SignalHandler(int, siginfo_t*, void*) /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Support/Unix/Signals.inc:426:38
 #3 0x00007a9f4c6e7330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
 #4 0x00007a9f4c7c928d syscall (/lib/x86_64-linux-gnu/libc.so.6+0x12728d)
 #5 0x00007a9f52f98ddb SignalHandler(int, siginfo_t*, void*) /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Support/Unix/Signals.inc:427:14
 #6 0x00007a9f4c6e7330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
 #7 0x00007a9f52a245bc operator= /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1523:69
 #8 0x00007a9f52a245bc operator= /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr.h:414:65
 #9 0x00007a9f52a245bc lldb_private::UnwindLLDB::GetRegisterContextForFrameNum(unsigned int) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/UnwindLLDB.cpp:472:16
#10 0x00007a9f52a28767 get /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1666:16
#11 0x00007a9f52a28767 _M_get /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1363:66
#12 0x00007a9f52a28767 operator-> /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1357:9
#13 0x00007a9f52a28767 lldb_private::RegisterContextUnwind::GetFullUnwindPlanForFrame() /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/RegisterContextUnwind.cpp:804:24
#14 0x00007a9f52a27427 lldb_private::RegisterContextUnwind::InitializeNonZerothFrame() /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/RegisterContextUnwind.cpp:645:5
#15 0x00007a9f52a24e33 IsFrameZero /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/RegisterContextUnwind.cpp:728:73
#16 0x00007a9f52a24e33 lldb_private::RegisterContextUnwind::RegisterContextUnwind(lldb_private::Thread&, std::shared_ptr<lldb_private::RegisterContextUnwind> const&, lldb_private::SymbolContext&, unsigned int, lldb_private::UnwindLLDB&) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/RegisterContextUnwind.cpp:80:7
#17 0x00007a9f52a238ba __shared_ptr<lldb_private::RegisterContextUnwind, void> /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1469:4
#18 0x00007a9f52a238ba shared_ptr<lldb_private::RegisterContextUnwind, void> /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr.h:214:25
#19 0x00007a9f52a238ba lldb_private::UnwindLLDB::GetOneMoreFrame(lldb_private::ABI*) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/UnwindLLDB.cpp:129:25
#20 0x00007a9f52a22dff __shared_ptr /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1532:20
#21 0x00007a9f52a22dff operator= /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1620:2
#22 0x00007a9f52a22dff operator= /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr.h:440:27
#23 0x00007a9f52a22dff lldb_private::UnwindLLDB::AddOneMoreFrame(lldb_private::ABI*) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/UnwindLLDB.cpp:337:15
#24 0x00007a9f52a23514 size /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/stl_vector.h:993:66
#25 0x00007a9f52a23514 lldb_private::UnwindLLDB::UpdateUnwindPlanForFirstFrameIfInvalid(lldb_private::ABI*) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/UnwindLLDB.cpp:315:37
#26 0x00007a9f52a22c15 lldb_private::UnwindLLDB::AddFirstFrame() /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/UnwindLLDB.cpp:0:3
#27 0x00007a9f52a2418f lldb_private::UnwindLLDB::DoGetFrameInfoAtIndex(unsigned int, unsigned long&, unsigned long&, bool&) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/UnwindLLDB.cpp:400:9
#28 0x00007a9f5299f011 GetFrameInfoAtIndex /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/include/lldb/Target/Unwind.h:53:12
#29 0x00007a9f5299f011 lldb_private::StackFrameList::FetchFramesUpTo(unsigned int, InterruptionControl) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/StackFrameList.cpp:435:41
#30 0x00007a9f5299e903 lldb_private::StackFrameList::GetFramesUpTo(unsigned int, InterruptionControl) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/StackFrameList.cpp:368:21
#31 0x00007a9f529a0a0f lldb_private::StackFrameList::GetFrameAtIndex(unsigned int) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/StackFrameList.cpp:634:7
#32 0x00007a9f529ed3a8 get /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1666:16
#33 0x00007a9f529ed3a8 lldb_private::Thread::GetSelectedFrame(SelectMostRelevant) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/Thread.cpp:276:34
#34 0x00007a9f5293e7cd operator bool /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1670:23
#35 0x00007a9f5293e7cd lldb_private::ExecutionContextRef::SetProcessPtr(lldb_private::Process*, bool) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/ExecutionContext.cpp:554:16
#36 0x00007a9f527a201b operator=<lldb_private::ExecutionContextRef> /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/optional:852:14
#37 0x00007a9f527a201b lldb_private::Debugger::DefaultEventHandler() /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Core/Debugger.cpp:2143:27
#38 0x00007a9f527a6509 std::_Function_handler<void* (), lldb_private::Debugger::StartEventHandlerThread()::$_0>::_M_invoke(std::_Any_data const&) /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/std_function.h:290:2
#39 0x00007a9f52862069 operator() /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/std_function.h:591:9
#40 0x00007a9f52862069 lldb_private::HostNativeThreadBase::ThreadCreateTrampoline(void*) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Host/common/HostNativeThreadBase.cpp:62:10
#41 0x00007a9f4c73eaa4 (/lib/x86_64-linux-gnu/libc.so.6+0x9caa4)
#42 0x00007a9f4c7cbc6c (/lib/x86_64-linux-gnu/libc.so.6+0x129c6c)

[JDevlieghere]

Hey @mgorny, any chance you could help me debug this. I can't reproduce this outside of the test run. My suspicion is that we're mucking with the process/thread/frame when we're doing the following of the child, but looking at the PR that adds support for (v)fork, I don't actually see where that might be happening.

[llvmbot]

@llvm/issue-subscribers-lldb

Author: Jonas Devlieghere (JDevlieghere)

The tests that set `target.process.follow-fork-mode = child` crash when computing the execution context on the event thread. I'm observing this in the context of https://github.com//pull/148994 and https://github.com//pull/159887.

The tests fail consistently in CI and relatively reliably when running the test suite in a Linux VM. However, running the exact same lldb invocation outside the test never reproduces the issue (not within 200 runs) and neither does it trip up ASan, UBSan or TSan.

The issue is reproducible by calling Debugger::GetSelectedContext from the default event handler.

"Pretty" backtrace:

 #<!-- -->0 0x0000000000264268 llvm::sys::PrintStackTrace(llvm::raw_ostream&amp;, int) /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Support/Unix/Signals.inc:834:13
 #<!-- -->1 0x0000000000261c75 llvm::sys::RunSignalHandlers() /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Support/Signals.cpp:105:18
 #<!-- -->2 0x0000000000265021 SignalHandler(int, siginfo_t*, void*) /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Support/Unix/Signals.inc:426:38
 #<!-- -->3 0x00007a9f4c6e7330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
 #<!-- -->4 0x00007a9f4c7c928d syscall (/lib/x86_64-linux-gnu/libc.so.6+0x12728d)
 #<!-- -->5 0x00007a9f52f98ddb SignalHandler(int, siginfo_t*, void*) /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Support/Unix/Signals.inc:427:14
 #<!-- -->6 0x00007a9f4c6e7330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
 #<!-- -->7 0x00007a9f52a245bc operator= /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1523:69
 #<!-- -->8 0x00007a9f52a245bc operator= /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr.h:414:65
 #<!-- -->9 0x00007a9f52a245bc lldb_private::UnwindLLDB::GetRegisterContextForFrameNum(unsigned int) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/UnwindLLDB.cpp:472:16
#<!-- -->10 0x00007a9f52a28767 get /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1666:16
#<!-- -->11 0x00007a9f52a28767 _M_get /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1363:66
#<!-- -->12 0x00007a9f52a28767 operator-&gt; /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1357:9
#<!-- -->13 0x00007a9f52a28767 lldb_private::RegisterContextUnwind::GetFullUnwindPlanForFrame() /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/RegisterContextUnwind.cpp:804:24
#<!-- -->14 0x00007a9f52a27427 lldb_private::RegisterContextUnwind::InitializeNonZerothFrame() /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/RegisterContextUnwind.cpp:645:5
#<!-- -->15 0x00007a9f52a24e33 IsFrameZero /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/RegisterContextUnwind.cpp:728:73
#<!-- -->16 0x00007a9f52a24e33 lldb_private::RegisterContextUnwind::RegisterContextUnwind(lldb_private::Thread&amp;, std::shared_ptr&lt;lldb_private::RegisterContextUnwind&gt; const&amp;, lldb_private::SymbolContext&amp;, unsigned int, lldb_private::UnwindLLDB&amp;) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/RegisterContextUnwind.cpp:80:7
#<!-- -->17 0x00007a9f52a238ba __shared_ptr&lt;lldb_private::RegisterContextUnwind, void&gt; /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1469:4
#<!-- -->18 0x00007a9f52a238ba shared_ptr&lt;lldb_private::RegisterContextUnwind, void&gt; /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr.h:214:25
#<!-- -->19 0x00007a9f52a238ba lldb_private::UnwindLLDB::GetOneMoreFrame(lldb_private::ABI*) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/UnwindLLDB.cpp:129:25
#<!-- -->20 0x00007a9f52a22dff __shared_ptr /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1532:20
#<!-- -->21 0x00007a9f52a22dff operator= /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1620:2
#<!-- -->22 0x00007a9f52a22dff operator= /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr.h:440:27
#<!-- -->23 0x00007a9f52a22dff lldb_private::UnwindLLDB::AddOneMoreFrame(lldb_private::ABI*) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/UnwindLLDB.cpp:337:15
#<!-- -->24 0x00007a9f52a23514 size /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/stl_vector.h:993:66
#<!-- -->25 0x00007a9f52a23514 lldb_private::UnwindLLDB::UpdateUnwindPlanForFirstFrameIfInvalid(lldb_private::ABI*) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/UnwindLLDB.cpp:315:37
#<!-- -->26 0x00007a9f52a22c15 lldb_private::UnwindLLDB::AddFirstFrame() /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/UnwindLLDB.cpp:0:3
#<!-- -->27 0x00007a9f52a2418f lldb_private::UnwindLLDB::DoGetFrameInfoAtIndex(unsigned int, unsigned long&amp;, unsigned long&amp;, bool&amp;) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/UnwindLLDB.cpp:400:9
#<!-- -->28 0x00007a9f5299f011 GetFrameInfoAtIndex /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/include/lldb/Target/Unwind.h:53:12
#<!-- -->29 0x00007a9f5299f011 lldb_private::StackFrameList::FetchFramesUpTo(unsigned int, InterruptionControl) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/StackFrameList.cpp:435:41
#<!-- -->30 0x00007a9f5299e903 lldb_private::StackFrameList::GetFramesUpTo(unsigned int, InterruptionControl) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/StackFrameList.cpp:368:21
#<!-- -->31 0x00007a9f529a0a0f lldb_private::StackFrameList::GetFrameAtIndex(unsigned int) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/StackFrameList.cpp:634:7
#<!-- -->32 0x00007a9f529ed3a8 get /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1666:16
#<!-- -->33 0x00007a9f529ed3a8 lldb_private::Thread::GetSelectedFrame(SelectMostRelevant) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/Thread.cpp:276:34
#<!-- -->34 0x00007a9f5293e7cd operator bool /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/shared_ptr_base.h:1670:23
#<!-- -->35 0x00007a9f5293e7cd lldb_private::ExecutionContextRef::SetProcessPtr(lldb_private::Process*, bool) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Target/ExecutionContext.cpp:554:16
#<!-- -->36 0x00007a9f527a201b operator=&lt;lldb_private::ExecutionContextRef&gt; /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/optional:852:14
#<!-- -->37 0x00007a9f527a201b lldb_private::Debugger::DefaultEventHandler() /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Core/Debugger.cpp:2143:27
#<!-- -->38 0x00007a9f527a6509 std::_Function_handler&lt;void* (), lldb_private::Debugger::StartEventHandlerThread()::$_0&gt;::_M_invoke(std::_Any_data const&amp;) /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/std_function.h:290:2
#<!-- -->39 0x00007a9f52862069 operator() /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/std_function.h:591:9
#<!-- -->40 0x00007a9f52862069 lldb_private::HostNativeThreadBase::ThreadCreateTrampoline(void*) /home/gha/actions-runner/_work/llvm-project/llvm-project/lldb/source/Host/common/HostNativeThreadBase.cpp:62:10
#<!-- -->41 0x00007a9f4c73eaa4 (/lib/x86_64-linux-gnu/libc.so.6+0x9caa4)
#<!-- -->42 0x00007a9f4c7cbc6c (/lib/x86_64-linux-gnu/libc.so.6+0x129c6c)

[mgorny]

To be honest, I don't really know where to start. I haven't debugged C++ for years (and really grateful I didn't have to). Were you perhaps able to attach a debugger while running the tests?